The production of cylindrical drill holes in materials such as concrete, brickwork, stone and the like using various techniques generally referred to as “drilling” is common. Often during above-ground, below-ground and street construction, for example, accurately positioned drill holes are needed, for example, for inserting fastening elements. Provided the holes are simple “passage holes”, the process is relatively simple, since there are no special requirements on the shape or geometry of the hole, apart from the nominal diameter. Dowel connections on the other hand impose additional requirements on the shape of the drilled hole.
There are already a number of special methods of making so-called undercut holes for receiving heavy-duty dowel connections. The purpose of these methods is to improve the performance of the hole by shaping it so that the force needed to pull the fastening element out of the hole is increased. This applies in general, but in particular to cracked concrete. With cracked concrete, it is necessary to prevent the dowel connection from yielding or subsiding owing to the cracking, which would reduce the load-bearing capacity. A positive connection is clearly an advantage, particularly for filigree elements where there is a limit to the spreading-apart forces which can be absorbed by the component.
Methods for making undercut holes, based, for example, on German Utility Model No 8808 256.3 and U.S. Pat. No. 4,993,894, first require the drilling of a standard drill hole with conventional drilling tools and machines. Then, in a second operation, by means of a geometrically not easily definable operation that is frequently called “swivelling or wobbling”, the cylindrical drill hole is widened at its inner end so that the inner end of an inserted dowel connector can become “wedged” in the hole. This enables a relatively defined positive connection to be made with the dowel connector.
Alternatively, there are special tools for making radial grooves in a standard drill hole and thus increasing its lateral roughness. These tools are used after drilling in a separate operation requiring a second working step. An example of such tools is disclosed in EP 0 795 677.
U.S. Pat. No. 4,989,681 and German Utility Model No 297 15 261 disclose the asymmetrical insertion of cutting edges, on the assumption that the axial force (action of pressure) and the cutting angle will result in an easily reproducible undercut.
In addition there are a number of devices on the market for working with movable, controlled cutting edges and blades. The difficulty usually is to remove the drilling dust on site and ensure unrestricted use under the rough conditions found at building sites. These methods and devices, hitherto in hand-guided versions, have not given satisfactory results with regard to the requirements of “widening” and “undercutting”, and therefore have not been commercially successful.